The present invention relates to a method and apparatus for determining the location of a mobile radio transmitter, especially a mobile radio transmitter positioned in the service area of a cellular communications telephone system.
Cellular telephone systems now provide ready access to wireless telephone communications. Cellular telephones operate in an analog system of frequency division multiple access (EDMA). Digital technologies, including time division multiple access (TDMA) or code division multiple access (CDMA), offer greater capacity and should give more individuals simultaneous access to cellular telephone services. In addition, cellular-like communications systems, such as personal communication systems (PCSs), will further increase the number of individuals with access to a wireless communication network.
A cellular-telephone or cell-like communication system involves a network of fixed base stations that provide an integrated communication service to a plurality of mobile transmitter/receiver ("transceiver") units, e.g., cellular telephones. The communications network attempts to communicate with each transceiver from the base station which provides the optimal communication. The optimal base station is usually, but not necessarily, the one nearest the mobile transceiver. To provide the optimal communications support, the network need not locate the geographic position of the mobile transceiver more accurately than needed to determine which base station to use.
The inability of existing communication networks for cellular-telephone or cell-like communication systems to accurately determine the location of a mobile transmitter is a major disadvantage in an emergency. For example, public safety officials in Los Angeles estimate that, today, a quarter of all who call the emergency number ("9-1-1") from a cellular telephone do not know where they are when they call. The time spent in finding their location delays emergency assistance units, for example, police or ambulance services, in providing assistance. Other studies indicate that in excess of sixty percent of traffic fatalities in the United States occur on rural roadways. Delays caused by uncertainty in location also exacerbate the inherently longer response times for providing emergency services in rural areas.
Monitoring mobile transceivers that are located on vehicles has advantages other than providing support for responses to requests for assistance. One such advantage is enabling the cost effective monitoring of traffic flow. Unplanned traffic incidents ("traffic jams") clog the highways with resulting deleterious effects on safety, environment, and economy. The volume of message traffic in a major metropolitan area is a type of collateral information, and it can be combined with observed location- and speed-related information and topographic information (e.g., road maps), to indicate which roads are passable and which are congested. However, traffic flow information, emergency services, and roadside assistance are not primaly reasons for establishing a communication system and thus are not currently provided by the system. The cost of adding equipment to the communications infrastructure to provide such information and services seems justifiable to communications companies only if it can be done using the most modest of infrastructure enhancements.
The problem of locating the position of a mobile radio transceiver has been solved in many ways for many years but in systems other than that of a cellular-telephone or cell-like communication system. No simple, low-cost solution has been found that is practical when applied to the wide-scale monitoring of mobile telephones. One practical difficulty in implementing any type of localization for mobile radio transceivers is the cost of the modifications either to the transceiver or to the communications network (infrastructure) that are needed to determine the location of the mobile transceiver. Any given transceiver would rarely, if ever, be used in placing a request for emergency or roadside assistance. Thus, the suppliers of transceivers and the operators of communications networks have little economic incentive to increase the complexity (and cost) of the transceivers or to install an extensive and expensive infrastructure to support such rarely used services absent government mandate. However unprofitable in the short term, emergency assistance and roadside assistance services have unquestionable value for providing and enhancing personal and public safety. Ameliorating the increasing incidence of violence and the related, growing concern for personal security with a mobile communications system is a worthy policy goal with the potential for realizing enormous benefit to subscribers, network operators, and the general public alike however, realizing the objective, even one so important and valuable, requires a practical, inexpensive infrastructure for uniquely identifying people requesting or reporting the need for assistance, communicating with them, and providing their locations to a responding assistant.
Today, techniques exist that provide partial and complex solutions to the problem of providing geographical locations with sufficient accuracy to aid emergency and roadside assistance personnel. For example, a radio transmitter may typically be attached to a vehicle which would enable the vehicle to be localized for purposes of protecting endangered cargo or persons, controlling the deployment of delivery trucks in an urban area or any other of a number of applications.
Several localization systems are presently commercially available. Some of the systems use navigational instruments such as ring laser devices. Others use magnetic field sensors that are sensitive to the earth's magnetic field. Yet another type uses radio beacons such as the LORAN-C system. While the systems perform satisfactorily, they are not suited for consumer use due to their inherent complexity and cost as well as the need for frequent reinitialization or calibration.
Techiniques exist for accurately determining one's position in applications other than that of providing emergency or roadside assistance. For example, the satellite-based Global Positioning System (GPS) allows determination of the location of the point of GPS signal reception with a special-purpose receiver for the wireless GPS signals that are broadcast from the satellites. However, obtaining the position of a communications transceiver by using GPS requires the mobile transceiver to include a GPS receiver. GPS receivers are expensive. Even if their cost were to be reduced through mass production, GPS receivers would still have to be integrated with all existing and future mobile transceivers. The cost associated with this solution seems to be prohibitive in view of the infrequency of use of the service and especially in terms of the large number of mobile transceivers for which the localization capability is desired.
Several attempts have been made over time to use terrestrial-based radio direction finding and positioning systems. One type of radio positioning system measures the time required for a radio signal to travel between a mobile transmitter station and fixed antenna locations. Time difference measurements are obtained by comparing the wide band signal wave forms transmitted from the mobile station with some form of pulsed amplitude modulation or specific coding modulation so that the timing resolution and related position resolution is proportional to the inverse of the signal bandwidth. Yet another radio positioning system uses time difference measurements obtained by comparing narrow band signal wave forms to obtain a difference in the phase of the received signal. The radio frequency signals have a wavelength comparable to the separation of the antenna sites and the ability to resolve the location of the mobile transmitter station is proportional to the wavelength of the signal. Either radio positioning system, however, requires synchronizing separated antenna sites. The synchronization requirement may be overcome by adding a special, known wave form to the radio signal. The waveforms received at each receiver, however, must be compared to determine the position of the mobile transmitter station. The common waveforms must not be distorted by any intervening interference.
A simpler method of determining the location of a mobile radio transmitter station involves passively monitoring its radio emissions, measuring the angle of arrival of a radio signal at a number of fixed locations, and then determining the area in which all the direction angles cross. One way to determine the direction angle is to electronically compare the difference in phase of the radio signal that is received by different antenna elements at a receiver site. Positioning the two antenna elements approximately 1/2 wavelength apart produces a narrow band intersignal phase difference which is proportional to the sine of the angle at which the signal is received.
However, with the simplest of approaches, radio localization does not take into consideration the distortions in apparent location caused by multipath interference (multipathing). Since most vehicles operate in an urban setting, it is highly desirable that any vehicle location system operate in an urban environment. A moving vehicle in an urban environment, however, seldom has a direct line-of-sight path to a receiver station. Rather, the propagation path contains many obstacles in the form of buildings and other structures, hill, and other vehicles which may be either landborne or airborne. Multipathing involves radio signals bouncing off of objects such as vehicles, buildings, hillsides, etc. The absence of a unique propagation path between the vehicle and the receiver station causes the instantaneous signal strength of any radio signal emitted from the vehicle to be highly variable at the receiver station. Indeed, it is known that the main propagation features of a radio signal in an urban environment are produced by multipath interference and shadowing of the direct line of sight path by intervening features of the terrain.
Multipath interference typically corresponds to so called Rayleigh signal fades. The signal fades occur because of plane wave interference and are separated by a ground distance of approximately one half wavelength apart. Multipath interference produces irregularly varying patterns of constructive and destructive interference as the mobile radio transmitter station moves through the service area which causes the radio frequency signal to fluctuate in amplitude, travel time and propagation direction. The exact position of any maxima or minima also depends on the wavelength of the radio frequency signal. Data which is transmitted during a deep fade typically is lost. Thus, multipath interference produces a complicated pattern of signal distortion which is an inherent characteristic of RF transmissions in an urban environment.
Without consideration of these effects, the apparent position of the transceiver will be distorted. Multipath propagation is common for short-wavelength, radio communications since relatively smaller objects can reflect substantial amounts of the transmitted signals, and it is especially common in cities with buildings reflecting the signals. The potential multipath-induced distortions in the apparent position of the mobile transceiver are therefore a problem that must be addressed in passively localizing radio emitters to support applications such as the provision of emergency or roadside assistance.
Multipath propagation conditions need not impede locating a transceiver when signal analysis and source localization procedures are used to ameliorate potential distortions in apparent position. For example, U.S. Pat. No. 4,728,959 issued to Maloney et al. ("the '959 patent"), which is hereby incorporated by reference in its entirety, demonstrates how direction finding procedures, by which the direction of the arrival of a signal can be estimated, can be applied with two or more receiving base stations.
The invention disclosed in the '959 patent combines the relative insensitivity of the phase angle differences of a radio signal to the signal distortions inherent in an urban environment with digital signal processing techniques to produce an accurate and economical way to locate a mobile transmitter station such as a mobile telephone in a cellular telephone network. Phase angle measurements indicative of the angle of direction of a mobile transmitter station from each of a plurality of land stations are obtained by translated Hilbert transformation and are processed to produce a probability density function. The probability density functions are combined after a chi-squared analysis to produce an area of uncertainty representing the position of the mobile transmitter station. The radio frequency signals emitted from the mobile transmitter station need have no special characteristics for the localization process. Thus, the invention disclosed in the '959 patent may easily work as an adjunct to an unrelated communications system such as a cellular telephone system.
In the preferred embodiment of the invention disclosed in the '959 patent, a transmission from the mobile transmitter station is received by at least two land stations. Receivers at at least two antenna elements identify the signal as coming from the target mobile transmitter station and reduce the frequency of the signal to correspond to the processing speed of digital signal processing circuitry. Processing units at the land stations determine complex phasor relationships between the antenna elements that represent the conjugate product of the signals in the two antenna elements corresponding to the phase of the radio signals in each antenna element and the direction angle to the mobile transmitter station from the land station. The phasor relationship is dependent on the amplitude of the recorded signal and weighted by a predetermined threshold so that the phasor is integrated with time to form a dynamically determined probability density function. The wavelengths of the signals preferably are short enough, e.g. HF, VHF and UHF, so that the antenna elements are closely spaced at a common land station and each element receives a signal of common phase evolution. The measured direction angles from at least two, and preferably more, land stations are combined at a control land station to determine the position of the mobile transmitter station.
The control land station applies a non-linear least-squares analysis to the raw data. Rather than selecting two or more "best" direction angle measurements to determine location, the invention disclosed in the '959 patent analyzes all available direction angle measurements for temporal and internal statistical, or chi-squared, consistency so as to mitigate multipath interference in the direction angle. The control land station generates an area of uncertainty from the density functions associated with the direction angle measurements.
For or a radio frequency system having a limited range of frequencies, both frequency division multiplexing and time division multiplexing are used to accommodate a number of mobile transmitter stations. The mobile transmitter station emits a signal which need have no special characteristic.
It is known that shadowing and multipath interference does not distort the frequency or the phase of a radio frequency signal as much as its amplitude and travel time. Thus, the relative insensitivity of frequency modulation to multipath interference has encouraged its use in cellular telephone systems which operate in, for example, the 800-900 MHz frequency bands. Shadowing and multipath interference nevertheless cause the apparent position of the mobile radio transmitter to randomly change with time so as to severely limit the accuracy and, hence, the applicability of the above mentioned phase difference technique of radio position locating to solving the problem of localizing a large number of vehicles in an urban environment. The invention disclosed in the '959 patent is thus well adapted to operate as an adjunct to existing cellular communications systems of a type that are presently serving most urban areas.
Using the passive monitoring of communication signals that is described in the '959 patent to determine location is an excellent application in that it allows for locating a mobile transceiver anywhere in a service area of a network having at least two receiving stations of known location. The direction finding approach is simpler, more accurate, and less costly than other approaches. However, the necessity of requiring joint reception of a common signal at multiple sites can increase the complexity and cost of this approach beyond what some cellular telephone or PCS companies are currently or may be willing to accept.
The systems described above rely on observed information derived from multiple, joint receptions of radio emissions, or on navigation information from devices extraneous to the communications transceiver. No system seeks to obtain location information from the combination of observed directional information, derived from communications radio receptions, with collateral information obtained, for example, from street maps. Therefore, it is an object of the present invention to provide a simple and effective way to identify and locate a mobile radio transceiver in any wireless communication system, including those already existing or that are contemplated, such as those for personal communication systems (PCSs), cellular telephones, specialized mobile radios (SMRs), and personal digital assistants (PDAs). It is an object of the present invention to provide an automatic location identification (ALI) and an automatic "number" identification (ANI) that facilitates national and international rural and urban emergency notification and personal security, and roadway monitoring by combining observed information derived from received radio emissions with collateral information derived from street maps, user descriptions, and other information sources.
The objectives of the present invention also include: providing a system in which location and identification are provided cheaply as adjuncts to communications for national and international wireless enhanced 9-1-1 (E 9-1-1) emergency and routine roadside assistance notification; estimating roadway speed and providing general transportation information such as traffic congestion and flow characterization; providing such capability in a system which is both relatively easy to deploy and inexpensive to construct; providing a system which has a transportable configuration and, therefore, can be used to temporarily monitor localized regions such as road construction areas or the localities of special events such as sporting competitions, conventions, or concerts; and providing a combination of processes and attributes to form an inexpensive yet robust system for localization and identification as an adjunct to a communications system.
Often, in addition to directional data that can be derived from received signal characteristics, other information is available or can be obtained that relates to the position of a mobile radio transceiver. For example, in a system designed to provide emergency roadside assistance, we may presume that the person requesting assistance is in a vehicle that is on or near a road. Such a presumption may be verified, for example, by asking the person placing the call if he or she is on a road. This type of additional geographic or topological information, called here "collateral information", is of a type that is normally available to a dispatcher. Combining collateral information with the directional information from even a single base station can define the location of a mobile radio transceiver well enough to make it possible to dispatch emergency and roadside assistance services. The derivation of the position of the transceiver solely from observed characteristics of its radio emissions received at multiple sites is not necessary, and the need for additional base stations thus becomes redundant. However, no proposal to date has sought to use such collateral information to make redundant the need for additional base stations.
The present invention provides an apparatus for locating a mobile radio transceiver in a cellular-telephone or cell-like communications system that comprises a sensor station of substantially known location, the sensor station having a directionally sensitive receiving antenna to receive a radio signal from the mobile transceiver, a signal characterization processing unit for determining a directional line of bearing from the sensor station to the mobile radio transceiver from the radio signal, a source of collateral information about the mobile transceiver, a multidimensional parametric correlation processing unit for determining, a probable position of the mobile transceiver from the line of bearing information and the collateral information, and an output indicative of the probable position of the mobile transceiver.
The present invention provides for locating a mobile radio transceiver in a cellular-telephone or cell-like communications system using a simplified system for passively monitoring signals emitted by the mobile transceiver. In this invention, the processing at a single receiving base station of known location determines a line of bearing (i.e., a direction angle from the receiving site) to the mobile transceiver location. This line of bearing is then combined with collateral information to determine the likely location of the transceiver. The present invention has particular applicability to roadway transportation in that it facilitates emergency (9-1-1) services and roadside assistance, and it permits the passive monitoring of traffic flow. The collateral information includes location information derived from other than radio location methods. Such information can include the topological information of a map of the roadways in the area of the base station, or other information such as derived speed, if any, of the transceiver, or information obtained from communications from the caller in person or from equipment at the caller's location.
The present invention does not require determining position by crossfixing a position using lines of bearing from two or more base stations; a single base station can be enough. This capability may have particular usefulness in a CDMA communications network in which increased capacity is obtained through dynamic power control so that only one base station is intended to receive a transceiver's emissions. Nevertheless, there is nothing in the present invention that precludes using more than one base station to further confirm the accuracy of a location or to permit locating mobile radio transceivers for which collateral information is not otherwise available. The ability to determine location from a single site has particular benefit for providing emergency assistance in that single site reception is applicable in more environments, requires less infrastructure, and offers greatly reduced cost. The present invention is particularly useful for monitoring traffic in rural areas that have fewer roads such that collateral information in the form of roadway topology better indicates the exact location of the mobile transceiver along the observed line of bearing. The present invention also provides a method and apparatus for locating a mobile radio transceiver in a wireless communications system, comprising a sensor station of substantially known location, a method and means for determining a line of bearing from the sensor station to the mobile radio communications transceiver, and a method and means for combining collateral information with the line of bearing to determine the location of the mobile radio transceiver.
The present invention has the advantage of being able to determine the location of a mobile radio transceiver without requiring embedding or integrating a special purpose device, such as a GPS receiver, with the mobile transceiver. Indeed, the present invention enables the localization of all existing cellular telephones. The cost of deploying a location system of the present invention is low. This low start up cost means that the system can be deployed faster so that consumers can realize the benefits sooner and at less expense.